Positive grid bias



Jan. 23, 1934.

w. VAN B. ROBERTS 1,944,574

POSITIVE GRID BIAS Filed oct, 28, 1929 INVENTOR WALTER VAN B. OBERTS BY 2 /w-v-Ol/ ATTORNEY Patented Jan. 23, 1934 UNITED STATES POSITIVE Walter van B. Roberts,

GRID BIAS Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 28,

10 Claims.

My present invention relates to space discharge devices, and more particularly to a novel method of, and means for, operating such devices.

Up to the present time, in operating space discharge tubes as amplifiers, it has been found necessary to employ high anode potentials. As is well known, the latter practice has been necessitated because the control electrode is maintained more negative than the cathode. Briefly, negative bias on the grid, or control, electrode has been deemed essential to avoid distortion or loss of ampliiication, the negative biasing of the grid resulting in making the input impedance to alternating currents between grid and cathode very high or iniinite thereby eliminating the cause of distortion or loss of amplification viz., the absorption of energy by the input circuit of the tube.

By experimentation, I have discovered a new method of achieving the same results, and devised novel means for securing these results. Accordingly, it is one of the main objects oi my present invention to provide a method of, and means for, operating a space discharge tube employing the usual control electrode, anode and cathode, which method consists in maintaining the input impedance to alternating currents very high, if not innite, by biasing the control electrode positive with respect to the cathode, and simultaneously employing a greatly reduced anode potential for the same anode current, whereby there results a considerable economy in operating expense and plate current power consumption.

Another important object of the invention is to provide a method of operating a spacefdischarge device as an amplifier, which device includes a cathode, control grid and one or more anodes, which consists in so adjusting the anode potentials, and in maintaining the control grid so positive, that the input resistance between the control grid and cathode is very large,l or even negative.

Another object of the invention is to provide a method of, and means for, operating a vacuum tube as an amplier, or even a detector, wherein the control electrode in the input circuit of the tube is so positively biased relative to the cathode that the input circuit does not absorb power from the source of signals, and a high amplification is secured with the employment of less anode potential than is necessary with a negatively biased control electrode tube producing the same amplication.

Still other objects of the invention are to improve generally the eflieiency of space discharge 1929. Serial No. 402,920 (o1. 17e-171) devices as amplifiers, and to provide a vacuum tube amplifier, or detector, which is not only economical to manufacture, but is reliable in operation and economical in power consumption.

'Ihe novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself,

however, as to both its organization and methodk of operation will best be understood by reference to the following description taken in connection with the drawing in which Ir have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

Inthe drawing, V

, Fig. 1 diagrammatically shows the invention basically,

Fig. 2 is a graphic analysis of the invention,

Fig. 3 diagrammatically shows a circuit embodying the invention used with a direct current house supply.

Referring to the accompanying drawing in which like characters of reference indicate the same parts in the diiierent views, there are shown in Fig. 2 curves representing the grid currents plotted against grid potential for variousvalues of plate potential. Such curves are easily obtained experimentally for any given tube by the use of the arrangement of Fig. 1. This figure embodies the invention, basically considered. The process is to set the plate potential Ep at some definite value, and then measure the grid current for various values of Eg. Repetition of this process for different p late potentials gives the curves of Fig. 2.

The input impedance between control grid an cathode is given by the expression will be infinite provided Eg is chosen large enough to bring the value of grid current onto the horizontal portion of the curve. .In this case, although Vthe grid is many volts positive and is drawing a steady current of the order of milliamperes, the alternating current resistance is infinite, or very large, so that no energy is abstracted from the alternating signal input voltage; hence, the input voltage is not reduced, nor

is the output current abnormally distorted.

Going still further, it is possible to choose a value of Epl, such as shown on the lowest curve ci Fig. 2, which gives a curve having a falling characteristic for a certain range of grid potentials. As is well known, this may be interpreted as a negative input resistance between control grid and cathode. If I do not go sofar as to produce oscillations when the tube is ied Vby a resonant circuit, I may obtain by a partial neutralization of the resistance of the signal input circuit an actual increase in input voltage, thus producing an even more efficient amplifier than if the grid were maintained negative, while at the 0 same time securing the advantage of operating the plate circuit on asmaller potential.

The latter advantage is of importance when it is desired to operate an amplifier from a source of --direct current, such as commonly supplied to dwellings in certain sections in New York city, the voltage of the house supply being only about 110 volts. The latter is not sumcient to get large output from modern tubes if they are operated with the grid strongly negative.V In Fig. 3, therefore, there is shown an arrangement adapted to be used in connection with a direct current house supply. v

Incoming signals are collected by a grounded i antenna A, and rectified by the simple circuit shown. Of course, one or more stages of radio amplincation may be employed before detection. The audio voltage resulting is impressed between the grid and cathode of tube l, a negative bias being maintained on the grid of this tube by means, forA example, of battery l0.

The electron flow in the anode circuit of this tube passes through the internal lament tovgrid circuit of tube'2, thus producing a fall of potential a between these electrodes which makes the grid of tube 2 more positive than its lament. By suitable adjustment of battery 1G, the plate current of tube 1 can be adjusted to produce a given positive potential Vof grid 8 with respect to cathode 10.

By moving slider 11 over the resistance 12, the latter being connected across the current supply, the plate potential of tube 2 may also be adjusted. By means of these two adjustments, it

A, is possible to obtain the proper plate and grid potentials for tube 2 to make the alternating current resistance between the grid 8 and nlament 10 either very large, or even negative as eX- plained heretofore.

i To by-pass high frequencies, a capacity C is E@ connected in shunt with the resistance 12, while a second capacity C1 is connected between the slider 11 and the cathodeend of the resistance 12.

The phones P are connected between the slider l1 and the anode oi? tube 2, it being understood that any other type of utilizationmeans, as a loud speaker, may be employed in place of the phones. ,Y

In the particular arrangement shown in Fig. 3,

l; the grid-filament direct current resistance is 'i 9 utilized in place of a coupling resistor to pass the current in the plate circuit of tube l, while its high alternating current resistance is utilized t0 produce large amplication of alternating signal Y. voltage. VIn other words, the system as far as f5 amplication is concerned behaves somewhat systems for carrying my invention into effect, vit will be apparent to one skilled in the art that my invention is by no means limited to the particuiar organizations shown and described, but that many modifications in the circuit arrangements, as well as in the apparatus employed may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. A method of operating an electron discharge amplifier tube including an anode, lcathode and grid which consists in constantly biasing said grid with a positive potential relative to the cathode, adjusting the anode potential, and maintaining the positive biasing potential on the grid at such a value relative to the cathode that the resistance component of impedance to alternating voltage between the grid and cathode is negative throughout the entire operation of the tube.

2. The method of operating a space discharge amplifier tube provided with a cathode, control electrode and an anode which consists in. applying a range of alternating currentpctentials between the cathode and control electrode, and maintaining the potentials of the anode and control electrode at such relative positive values that the input alternating current impedance ol the amplifier is high over said range.

3. The method of operating a vacuum tube as an amplifier, the tube having a' cathode control grid and anode, which consists in so adjusting the anode potential, and in maintaining` the control grid potential so positive, that the alternating current input resistance between the control 120 grid and cathode is negative throughout the range of alternating current voltages applied between the cathode and grid of the tube.

4. In combination with an amplifier space discharge tube having Vits anode and cathode included in a circuit energized from a source of current, means for 'constantly impressing upon the control electrode a potential more positive than that of the tube cathode, said potential being so much more positive than that of the tube cathode that the alternating current impedance between the control electrode and cathode is maintained negative throughout the operation of the tube.

5. The method of operating an ampliiier tube provided with an input circuit including a grid and cathode, and an output circuit including an anode, which consists in applying alternating current potentials to the input circuit, and simultaneously maintaining the grid so positively biased that the alternating current input impedance of the tube is substantially infinite for all applied potentials.

6. An amplifier comprising an electron discharge tube includingan anode, cathode and control electrode, means for biasing said control electrode with a positive potential relative to the cathode, means for applying a more positive potential to said anode, and additional meansr for adjusting the positive biasing potential of said 150 control electrode through a range of values such that the resistance component, of impedance to alternating voltage between the control electrode and cathode, varies from negative to high values throughout a predetermined range of applied alternating current potentials.

'7. In an amplication system comprising, in eiect, a pair of thermionic devices connected so that the output circuit of one thereof includes the grid-cathode path of the other thereof, means for operating the first of said devices at a negative mean grid bias with respect to its cathode, means for operating the other of said devices at a positive mean grid bias with respect to its cathode and a common source of space current for both of said devices.

8. In an amplification system comprising, in effect, a pair of cascaded thermionic devices connected so that the output circuit of one thereof includes the grid cathode path of the other thereof, means for operating the rst of said devices at a negative grid bias with respect to its cathode, means for operating the other of said devices at a positive mean grid bias with respect to its cathode, a common source of anode potential for both of said devices, a potentiometer connected across said source and means for tapping the anode potential for the first of said devices at a point on said potentiometer, means for tapping the anode potential for the second named device at another point of said potentiometer, the rst of said tapping points being at a higher potential than the second thereof.

9. In an amplification system comprising, in eiIect, a pair of cascaded thermionic devices each thereof having an anode, a cathode and at least one grid electrode, said two devices being connected so that the output circuit of one thereof includes the grid cathode path of the other thereof, a biasing means connected in the input circuit of the first device for operating said first device at a negative grid bias with respect to its cathode, said grid cathode path of the second named device being included in the output circuit of the other device as regards current flow therein so that the second named device is operated at a positive mean grid bias with respect to its cathode, a source of space current for both of said devices comprising a resistance shunted across a source of direct current, an output circuit for the second named device including a translating means and a portion of said resistance in series, a by-pass condenser connected from the cathode of the second 'named device to a point in the output circuit thereof between the translating means and the resistance.

10. In combination with a source of signal modulated carrier energy, a detector circuit for deriving from the modulated carrier energy the signal energy, an amplifier for the derived signal energy comprising, in eifect, a pair of space discharge devices each thereof having anode, cathode and grid electrodes inter-related so as to form input and output circuits for said devices, the output circuit of the first of the cascaded devices including the input circuit grid-cathode path of the other thereof and a source of space current in series whereby the output current of the first device iiows through the grid-cathode path of the other, the impedance of said path maintaining the grid of the second device at a positive mean potential with respect to its cathode, a source of anode potential for the second device and means for adjusting the anode potential applied to the second device with relation to the said positive mean potential thereof to such a value that the grid current-grid potential characteristic curve of the second named device includes a portion wherein an increase in grid voltage results in decreased grid current.

WALTER VAN B. ROBERTS. 

